1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine provided with a noble metal chip in an ignition section.
2. Description of the Related Art
These days, engines are becoming of more and more high performance, of supercharging design, and adapted to be of a lean-burn type and have higher compression ratios, and accordingly, an increase in the discharge voltage of a spark plug is unavoidable, which might result in the risk of exceeding the voltage generated at power source. Further, engines free from maintenance are coming into great demand, and it is also highly demanded that the life of the spark plugs be prolonged.
Conventionally, in order to satisfy a reduction in the discharge voltage and an increase in the life of the spark plugs as stated above, a spark plug has been proposed in Japanese Patent Publication No. 63-62870. Such a spark plug is constructed such that an elongated bar-shaped noble metal chip which is extremely excellent in wear-resisting properties is attached to a tip of an electrode material forming at least one of a center electrode and an earth electrode.
Such a noble metal chip is made of a Pt--Ir alloy essentially consisting of 70-90% platinum and 30-10% iridium, or a Pt--Ni alloy essentially consisting of 80-90% platinum and 20-10% nickel. The electrode material is made of a nickel-base heat-resisting alloy.
When the noble metal chip is secured to the tip of the electrode material as stated above, a recess which is slightly wider than the diameter of the noble metal chip is first formed on the tip of the center electrode, and then, the noble metal chip is fit into such a recess. Subsequently, they are pressed, and also welded by using a laser or an electron beam, or the like, whereby an end of the noble metal chip is expended to have the shape of a flange and firmly welded to the recess of the center electrode.
The noble metal chip uses expensive materials such as a Pt--Ir alloy or a Pt--Ni alloy. However, a large portion of the noble metal chip has to be disposed of, although it is intact, when the life of the spark plug comes to an end.
More specifically, only a small portion of the noble metal chip wears due to discharge voltage. However, when a spark gap exceeds a predetermined size (the gap size occurring when the discharge voltage exceeds the voltage generated at power source), that is, when the life of the spark plug comes to an end, a large portion of the noble metal chip has to be disposed of together with the spark plug. This is undesirable not only for economical reasons but also from the respect of protecting the natural environment.
Also, there is a big difference between the coefficient of linear expansion of the noble metal chip and that of a Nickel-base heat-resisting alloy composing the electrode material.
Thus, after the spark plug is used in an engine at a high temperature over a long period, a crack 91 might occur due to thermal stress at a noble metal chip 9 developed in the vicinity of a contact surface 98 between an electrode material 8 and the noble metal chip 9, as illustrated in FIG. 13. When the crack 91 becomes greater, there is caused such a risk that the noble metal chip 9 will peel and fall off from the electrode material 8, as shown in FIG. 14.
Further, when a cooling load is applied to the engine at a high temperature, tensile stress is produced in the electrode material having the greater coefficient of linear expansion due to the thermal stress during cooling. At this time, a swelling 83 is caused at the electrode material 8 due to plastic deformation, as illustrated in FIG. 15. Such a swelling 83 is further expanded by repeating the application of the cooling load.
Moreover, stress corrosion occurs at the electrode material due to such an excessive stress, and when the corrosion is extremely aggravated, the noble metal chip might peel and fall off as shown in FIG. 14, which results in shortening the life of the spark plug.